1. Technical Field
This invention relates to the metallurgy of iron and zinc, and particularly to the separation of zinc from iron and other metals found in the dusts produced in the electric arc furnace (EAF) steel making process. The invention also applies to dusts arising from a foundry, blast furnace or other source. In addition to zinc, metals and/or metal oxides which may be recovered by the process of this invention include cadmium, lead and antimony. Additional applications may also be found in the recovery of zinc from ore concentrates, recovered or waste metallic powders from coatings industries, wastewater treatment solids from high zinc wastewaters such as are found in the munitions industry, and the like.
The problem encountered in the processing of electric arc furnace dusts is one of finding an economical method of separating the zinc from the remainder of the dust which consists mainly of iron, manganese, nickel, silicon and smaller quantities of other elements. Part of the problem in removing zinc from the dust stems from the fact that the zinc is often combined with the iron in the form of zinc ferrite. Removing zinc from a bimetallic compound is a more difficult task than one in which zinc and iron are present as discrete compounds; for example, a mixture of zinc oxide and iron oxide. The objective of a zinc/iron separation process, such as the invention disclosed herein, is to remove and recover the zinc and to obtain an iron containing product which can be safely disposed or which can be recycled to the steel making process without further purification or separation.
This invention describes an economical process which uses hydrogen to reduce zinc oxide to zinc metal, sweeps the zinc metal from the furnace dust or other source, reoxidizes the zinc to zinc oxide with simultaneous regeneration of the hydrogen, recycles the hydrogen to the process, recovers the separated zinc oxide, and produces a zinc-depleted, iron-containing dust which can be recycled back to the steelmaking process.
2. Prior Art
In the ores, dusts and powders to which this invention is directed, the zinc is substantially present in the form of zinc oxide. The prior art methods of separation involve reducing the zinc oxide to zinc metal at a temperature at which zinc metal has a vapor pressure sufficient to permit it to be sublimed or swept from the system by a gas stream. Pure zinc boils at 907.degree. C. and has a vapor pressure of 0.1 atmosphere at 750.degree. C. It is known to use carbon or carbon containing compounds such as methane or carbon monoxide as a reductant for zinc. Though hydrogen could have been used instead of methane or carbon monoxide in the prior art processes, it has not been so favored because its use is thermodynamically less favorable, higher temperatures are required and, until the present invention, the hydrogen could not be regenerated and recycled. The inability to regenerate and recycle hydrogen has been the primary drawback because of the higher associated operating costs.
U.S. Pat. Nos. 4,393,423 and 4,053,301 to Stephens, describe a process which uses hydrogen as a reductant, but the hydrogen is used in conjunction with other reducing gases such as methane and carbon monoxide. Stephens describes his invention in the '423 patent as involving a carbiding step to produce iron carbide and distillable zinc from a flue dust containing iron and zinc compounds, including zinc ferrite. Stephens teaches the use of hydrogen in conjunction with a carbon containing gas, but does not disclose the use of hydrogen gas as the sole or principal reducing agent, nor does he suggest a method in which the zinc is reoxidized and the hydrogen is regenerated and recycled. In both Stephens patents, hydrogen is used on a net once through basis, the hydrogen combining with oxygen to form water which is condensed and removed from the recycle reducing gas stream. The present invention differs from the Stephens invention in that hydrogen is the sole reducing agent and that the hydrogen is regenerated by reaction of the zinc metal swept from the flue dust with the water during a reoxidation step. The regeneration of the hydrogen is accomplished in the overall process of the present invention in two steps. In the first step, hydrogen and zinc oxide react at one temperature to form distillable zinc metal vapor and water vapor. In the second step, the zinc metal vapor and water vapor recombine at a lower temperature to reform zinc oxide, which is removed by filtration or other means, and hydrogen, which is recycled to the process.
In addition to the Stephens patents, other prior patent art pertinent to the invention are U.S. Pat. Nos. 4,318,736 to Driemeger; 3,975,118 to Harvey et al.; 3,449,117 to Derham; 4,131,451 to Lakernick et al.; 3,140,168 to Halley et al.; 3,288,590 to Keith et al.; and 1,946,601 to Hansgrig. Driemeger suggests using a hydrogen rich gas (H.sub.2 and CO) to sweep iron ore dust (essentially pure iron oxide) into a steel making furnace where the iron ore dust will be "spontaneously reduced" to metallic iron. Harvey et al. uses once through carbon monoxide to reduce and sweep zinc from a reactor. Derham uses an electrothermal heating means to purify zinc metal distributed on a column of carbonaceous material. Lakernick et al. uses a stream of nitrogen gas to sweep zinc from a slag already treated with a carbonaceous reducing agent. Halley et al. reduces iron ore to iron using electric arc produced atomic hydrogen. Keith et al. describes the use of hydrogen to reduce metal oxides, but does not suggest using the method to separate one metal from another metal, does not mention metal distillation or volatilization and reoxidation, and does not provide for hydrogen regeneration and recovery. Finally, Hansgrig describes reducing magnesium oxide with carbon in a hydrogen atmosphere and distilling or volatilizing the magnesium in the same atmosphere to prevent the formulation of a magnesium oxide film on the purified metal. As with the other processes which use hydrogen, Hansgrig does not disclose regenerating and recycling the hydrogen.
In addition to the patent art, the technical literature of the basic metals industries is replete with articles on the uses and treatment of electric arc furnace (EAF) dust. The field was thoroughly reviewed in a Final Report entitled, "Electric Arc Furnace Dust--Disposal, Recycle and Recovery," which was prepared for the Center For Metal Production, Mellon Institute, Carnegie-Mellon University by the Research Department of Bethlehem Steel Corporation in 1985. The Report reviewed the then current and emerging EAF dust technology; especially technology which focused on removing zinc from the EAF dust. The technologies reviewed--plasma arc reactors, melt injection reactors, solid or liquid phase selective reduction, Waela Kiln, the HTR (Himezi Tekko Rifain) process, electrothermic smelting, and the SKF Plasmadust process--all use carbon compounds, i.e., coke, as the reductant. Technologies which did not use carbon compounds were hydrometallurgical, electrowinning and chlorination processes. These last technologies seriously suffered from cost or environmental problems, and in addition, did not recover zinc from zinc ferrite species present in the EAF dust.
A more recent review of the methods for recovering or disposing of EAF dust appears in the August, 1987 issue of Iron and Steelmaking, a publication of the Iron and Steel Society. The article, "So Little Time, So Many Choices" reviewed nine methods for treating EAF dust. With the exception of a fixation method that uses silicate and silica settling agents to stabilize the EAF dust in glass blocks, all the technologies used carbon compounds as the reductant.
It is an object of the present invention to provide a method for separating and recovering metals from a substance containing a mixture of metals or metal oxides. It is a further object of this invention to provide a method for such separation and recovery which regenerates and recycles the reducing gas which is used to reduce and separate the desired metals from said mixture of metals or metal oxides. It is a special object of this invention to provide a method for separating, volatilizing and recovering zinc (in the form of zinc oxide) from a mixture of metal or metal oxides, and especially from EAF dusts.